1. Field of the Invention
The present invention relates to a structure of a lens barrel that employs a frame member movable along an optical axis.
2. Description of the Related Art
A generally employed lens barrel provided with a movable frame that is movable along the optical axis and a cam ring used for driving to move forward/backward the movable frame has a structure in which three single lined cam grooves formed at the cam ring side are respectively engaged with three cam followers formed at the movable frame side. The movable frame is driven to move forward/backward by driving the cam ring.
Japanese Unexamined Patent Application Publication No. 2004-233928 discloses the structure of the lens barrel that allows increase in a distance of forward movement of the movable frame member, and reduction in the length of the lens barrel in a retracted state. The disclosed lens barrel includes a cam ring that is driven to rotate, and a movable ring that is driven to move forward/backward while having the rotation restricted. FIGS. 10 and 11 show development views each showing an engagement of a cam follower of a movable ring 80 with a cam groove of the cam ring 110 in the lens barrel. FIG. 10 shows a wide state, and FIG. 11 shows a tele state, respectively.
Referring to FIGS. 10 and 11, three double rows (pairs) of cam grooves 110a1 and 110a2 are provided on the inner periphery of the cam ring 110. Three pairs of cam followers 80b1 and 80b2 are provided on the outer periphery of the movable ring 80. When the cam ring 110 is driven to rotate, the cam followers 80b1 and 80b2 are driven to move forward/backward along the cam grooves 110a1 and 110a2 such that the movable ring 80 is moved to a wide position and further to a tele position relative to the cam ring 110.
As shown in FIG. 10, one of the cam grooves, that is, 110a2 disappears at an intermediate position of the cam groove trace at a position to the front and rear of the wide position. During the driving at the position to the front and rear of the wide position, one of the cam followers, that is, 80b2 is disengaged from the cam groove 110a2 so as to be brought into a free state. As shown in FIG. 11, the other cam groove 110a1 disappears around the tele position, and the other cam follower 80b1 is disengaged from the cam groove 110a2 so as to be brought into a free state. During other driving period than at the position to the front and rear of the wide position and around the tele position, the cam followers 80b1 and 80b2 are engaged with the cam grooves 110a1, 110a2, respectively at the same time (see cam followers 80b1′, 80b2′).
Accordingly, the aforementioned lens barrel having the double row cam follower structure allows increase in the forward/backward movement distance to be longer in spite of the use of the cam ring that is shorter than the one using the mechanism for moving forward/backward the normal lens barrel that engages the single row cam groove with a single cam follower.